1=========================================
2Libc++ 15.0.0 (In-Progress) Release Notes
3=========================================
4
5.. contents::
6   :local:
7   :depth: 2
8
9Written by the `Libc++ Team <https://libcxx.llvm.org>`_
10
11.. warning::
12
13   These are in-progress notes for the upcoming libc++ 15 release.
14   Release notes for previous releases can be found on
15   `the Download Page <https://releases.llvm.org/download.html>`_.
16
17Introduction
18============
19
20This document contains the release notes for the libc++ C++ Standard Library,
21part of the LLVM Compiler Infrastructure, release 15.0.0. Here we describe the
22status of libc++ in some detail, including major improvements from the previous
23release and new feature work. For the general LLVM release notes, see `the LLVM
24documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
25be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.
26
27For more information about libc++, please see the `Libc++ Web Site
28<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.
29
30Note that if you are reading this file from a Git checkout or the
31main Libc++ web page, this document applies to the *next* release, not
32the current one. To see the release notes for a specific release, please
33see the `releases page <https://llvm.org/releases/>`_.
34
35What's New in Libc++ 15.0.0?
36============================
37
38Implemented Papers
39------------------
40
41- P0627R6 (Function to mark unreachable code)
42- P1165R1 (Make stateful allocator propagation more consistent for ``operator+(basic_string)``)
43- P0674R1 (Support arrays in ``make_shared`` and ``allocate_shared``)
44- P0980R1 (Making ``std::string`` constexpr)
45- P2216R3 (std::format improvements)
46- P0174R2 (Deprecating Vestigial Library Parts in C++17)
47- N4190 (Removing auto_ptr, random_shuffle(), And Old <functional> Stuff)
48- P0154R1 (Hardware inference size)
49- P0618R0 (Deprecating <codecvt>)
50
51- Marked the following papers as "Complete" (note that some of those might have
52  been implemented in a previous release but not marked as such):
53
54    - P1207R4 (Movability of Single-pass Iterators);
55    - P1474R1 (Helpful pointers for ``ContiguousIterator``);
56    - P1522R1 (Iterator Difference Type and Integer Overflow);
57    - P1523R1 (Views and Size Types);
58    - P1456R1 (Move-only views);
59    - P1870R1 (``forwarding-range`` is too subtle);
60    - P1878R1 (Constraining Readable Types);
61    - P1970R2 (Consistency for ``size()`` functions: Add ``ranges::ssize``);
62    - P1983R0 (Wording for GB301, US296, US292, US291, and US283).
63
64New Features
65------------
66
67- `pop_heap` now uses an algorithm known as "bottom-up heapsort" or
68  "heapsort with bounce" to reduce the number of comparisons, and rearranges
69  elements using move-assignment instead of `swap`.
70
71- Libc++ now supports a variety of assertions that can be turned on to help catch
72  undefined behavior in user code. This new support is now separate from the old
73  (and incomplete) Debug Mode. Vendors can select whether the library they ship
74  should include assertions or not by default. For details, see
75  :ref:`the documentation <assertions-mode>` about this new feature.
76
77- The implementation of the function ``std::to_chars`` for integral types has
78  moved from the dylib to the header. This means the function no longer has a
79  minimum deployment target.
80
81- The format functions (``std::format``, ``std::format_to``, ``std::format_to_n``, and
82  ``std::formatted_size``) now validate the format string at compile time.
83  When the format string is invalid this will make the code ill-formed instead
84  of throwing an exception at run-time.  (This does not affect the ``v``
85  functions.)
86
87
88API Changes
89-----------
90
91- The ``_LIBCPP_ABI_UNSTABLE`` macro has been removed in favour of setting
92  ``_LIBCPP_ABI_VERSION=2``. This should not have any impact on users because
93  they were not supposed to set ``_LIBCPP_ABI_UNSTABLE`` manually, however we
94  still feel that it is worth mentioning in the release notes in case some users
95  had been doing it.
96
97- The header ``<experimental/filesystem>`` has been removed. Instead, use
98  ``<filesystem>`` header. The associated macro
99  ``_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`` has also been removed.
100
101- Libc++ is getting ready to remove unnecessary transitive inclusions. This may
102  break your code in the future. To future-proof your code to these removals,
103  please compile your code with ``_LIBCPP_REMOVE_TRANSITIVE_INCLUDES`` defined
104  and fix any compilation error resulting from missing includes.
105
106- The ``<algorithm>``, ``<array>``, ``<optional>``, ``<unordered_map>`` and ``<vector>``
107  headers no longer transitively include the ``<functional>`` header. If you see compiler
108  errors related to missing declarations in namespace ``std``, make sure you have the
109  necessary includes.
110
111- The integer distributions ``binomial_distribution``, ``discrete_distribution``,
112  ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``,
113  and ``uniform_int_distribution`` now conform to the Standard by rejecting
114  template parameter types other than ``short``, ``int``, ``long``, ``long long``,
115  (as an extension) ``__int128_t``, and the unsigned versions thereof.
116  In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
117
118- The C++14 function ``std::quoted(const char*)`` is no longer supported in
119  C++03 or C++11 modes.
120
121- Setting a custom debug handler with ``std::__libcpp_debug_function`` is not
122  supported anymore. Please migrate to using the new support for
123  :ref:`assertions <assertions-mode>` instead.
124
125- ``vector<bool>::const_reference``, ``vector<bool>::const_iterator::reference``
126  and ``bitset::const_reference`` are now aliases for `bool` in the unstable ABI.
127
128- The ``_LIBCPP_DEBUG`` macro is not supported anymore. It will be honoured until
129  LLVM 16, and then it will be an error to define that macro. To enable basic
130  assertions (previously ``_LIBCPP_DEBUG=0``), please use ``_LIBCPP_ENABLE_ASSERTIONS=1``.
131  To enable the debug mode (previously ``_LIBCPP_DEBUG=1|2``), please ensure that
132  the library has been built with support for the debug mode, and it will be
133  enabled automatically (no need to define ``_LIBCPP_DEBUG``).
134
135- The ``_LIBCPP_DISABLE_EXTERN_TEMPLATE`` macro is not honored anymore when defined by
136  users of libc++. Instead, users not wishing to take a dependency on libc++ should link
137  against the static version of libc++, which will result in no dependency being
138  taken against the shared library.
139
140- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION`` macro has been added to allow
141  re-enabling the ``allocator<void>`` specialization. When used in conjuction with
142  ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS``, this ensures that the members of
143  ``allocator<void>`` removed in C++20 can be accessed.
144
145- The experimental versions of ``boyer_moore_searcher`` and ``boyer_moore_horspool_searcher``
146  will be removed in LLVM 17. You can disable the deprecation warnings by defining
147  ``_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_SEARCHERS``.
148
149- ``std::function`` has been removed in C++03. If you are using it, please remove usages
150  or upgrade to C++11 or later. It is possible to re-enable ``std::function`` in C++03 by defining
151  ``_LIBCPP_ENABLE_CXX03_FUNCTION``. This option will be removed in LLVM 16.
152
153- ``unary_function`` and ``binary_function`` are no longer available in C++17 and C++20.
154  They can be re-enabled by defining ``_LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION``.
155  They are also marked as ``[[deprecated]]`` in C++11 and later. To disable deprecation warnings
156  you have to define ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS``. Note that this disables
157  all deprecation warnings.
158
159- The contents of ``<codecvt>``, ``wstring_convert`` and ``wbuffer_convert`` have been marked as deprecated.
160  To disable deprecation warnings you have to define ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS``. Note that this
161  disables all deprecation warnings.
162
163ABI Changes
164-----------
165
166- The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
167  emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
168  ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
169  will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
170
171- On FreeBSD, NetBSD, DragonFlyBSD and Solaris, ``std::random_device`` is now implemented on
172  top of ``arc4random()`` instead of reading from ``/dev/urandom``. Any implementation-defined
173  token used when constructing a ``std::random_device`` will now be ignored instead of
174  interpreted as a file to read entropy from.
175
176- ``std::valarray``'s unary operators ``!``, ``+``, ``~`` and ``-`` now return an expression
177  object instead of a ``valarray``. This was done to fix an issue where any expression involving
178  other ``valarray`` operators and one of these unary operators would end up with a dangling
179  reference. This is a potential ABI break for code that exposes ``std::valarray`` on an ABI
180  boundary, specifically if the return type of an ABI-boundary function is ``auto``-deduced
181  from an expression involving unary operators on ``valarray``. If you are concerned by this,
182  you can audit whether your executable or library exports any function that returns a
183  ``valarray``, and if so ensure that any such function uses ``std::valarray`` directly
184  as a return type instead of relying on the type of ``valarray``-expressions, which is
185  not guaranteed by the Standard anyway.
186
187Build System Changes
188--------------------
189
190- Support for standalone builds have been entirely removed from libc++, libc++abi and
191  libunwind. Please use :ref:`these instructions <build instructions>` for building
192  libc++, libc++abi and/or libunwind.
193
194- The ``{LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE``, ``{LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT`` and
195  ``{LIBCXX,LIBCXXABI,LIBUNWIND}_GCC_TOOLCHAIN`` CMake variables have been removed. Instead, please
196  use the ``CMAKE_CXX_COMPILER_TARGET``, ``CMAKE_SYSROOT`` and ``CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN``
197  variables provided by CMake.
198
199- When building for Windows, vendors who want to avoid dll-exporting symbols from the static libc++abi
200  library should set ``LIBCXXABI_HERMETIC_STATIC_LIBRARY=ON`` when configuring CMake. The current
201  behavior, which tries to guess the correct dll-export semantics based on whether we're building
202  the libc++ shared library, will be removed in LLVM 16.
203
204- Previously, the C++ ABI library headers would be installed inside ``<prefix>/include/c++/v1``
205  alongside the libc++ headers as part of building libc++. This is not the case anymore -- the
206  ABI library is expected to install its headers where it wants them as part of its own build.
207  Note that no action is required for most users, who build libc++ against libc++abi, since
208  libc++abi already installs its headers in the right location. However, vendors building
209  libc++ against alternate ABI libraries should make sure that their ABI library installs
210  its own headers.
211
212- The legacy testing configuration is now deprecated and will be removed in the next release. For
213  most users, this should not have any impact. However, if you are testing libc++, libc++abi or
214  libunwind in a configuration or on a platform that used to be supported by the legacy testing
215  configuration and isn't supported by one of the configurations in ``libcxx/test/configs``,
216  ``libcxxabi/test/configs`` or ``libunwind/test/configs``, please move to one of those
217  configurations or define your own.
218
219- The ``LIBCXX_ENABLE_DEBUG_MODE_SUPPORT`` CMake configuration is not supported anymore. If you
220  were disabling support for the debug mode with that flag, please use ``LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=OFF``
221  instead.
222
223- MinGW DLL builds of libc++ no longer use dllimport in their headers, which
224  means that the same set of installed headers works for both DLL and static
225  linkage. This means that distributors finally can build both library
226  versions with a single CMake invocation.
227